Radio communication channels such as HF, VHF and UHF introduce distortion in the form of multipath, fading and other types of interference into the originally transmitted signal. Transmission distortion can be caused by, for example, multipath reception of the original signal, group delay distortion, noise amplitude distortion, interference and fading. A symptom of these distortions is intersymbol interference. Intersymbol interference occurs if modulation bandwidth exceeds the coherent bandwidth of the radio channel, which in turn causes the modulation pulses to spread in time to adjacent symbols. Intersymbol interference can also be caused by the radio channel exhibiting time and frequency dispersion (e.g., delay spread and Doppler spread) due to the presence of signal reflectors/scatterers in the environment or the relative motion of transmitter and receiver.
Intersymbol interference causes bit errors at the receiver which distorts the intended message content. To address transmission channel distortion adaptive equalizers or recursive filters have been included in the receivers. FIG. 1 schematically illustrates signal processing steps of a conventional receiver. Referring to FIG. 1, receiver 100 includes an antenna 110 for receiving the signal; radio receiver filter 120 for limiting the bandwidth of the incoming signal (typically to 3 KHz for an HF signal); digital down-conversion 130 for converting signal from 1800 Hz carrier to baseband (for an HF signal); digital low pass filter 140; demodulator 150 (with one or more equalizer); deinterleaver 160 and decoder 170 for forward error correction. The block interleaver typically has several different user selectable lengths to allow selection of proper block size for current channel conditions.
Adaptive equalizers are widely used in demodulator 150 because of their ability to continually change their equalization characteristics (filter coefficients) in response to the time-varying nature of the channel distortion. A commonly used equalizer is a decision feedback equalize (“DFE”). The principle of operation of a DFE is that once an information symbol has been detected and decided upon, the amount of intersymbol interference caused by this symbol can be estimated and removed while the subsequent symbols are being processed.
FIG. 3 is a schematic representation of a conventional DFE. Referring to FIG. 3, equalizer 300 is shown to have both a feedforward filter (“FFF”) and a feedback filter (“FBF”). The FBF is driven by the output of the detector 340 and each of the coefficients 350 is adjusted to cancel intersymbol interference on the current symbol from the previously-detected symbols. Equalizer 300 has N filter taps in the FFF and three taps in the FBF. Shift registers are conventionally used as delay elements 315 and 345, and the delay period is typically equivalent to a symbol period Ts. Tap weights 320 for the FFF and tap weights 350 for the FBF can be determined based on known algorithms such as zero forcing (“ZF”), the least means squares (“LMS”), the recursive least squares (“RLS”), Minimum Mean Square Error Criterion, to name a few.
The DFE 300 filter coefficients can be computed based on the estimate of the channel. The operation of the adaptive equalizer may include training and tracking. During the training process, the transmitter sends a fixed-length training sequence (or the “known symbols”) which is known apriori by the receiver. This is done so that the receiver's equalizer can continually adapt in order to minimize the bit error rate (“BER”) and to acquire initial filter coefficients even in the worst possible channel conditions. Once training is completed, the filter coefficients are near their optimal values and ready to receive the actual message (herein referred to as the “unknown data symbols”).
A DFE can operate in one of two ways. In one mode (the so-called data-directed mode) decisions being made can be used to adapt filter coefficients. Alternatively, the known data symbols can be used to compute channel estimates and interpolated channel estimates. These estimates are then used to compute the filter coefficients of DFE using known algorithms. An advantage of the second approach is that it removes the added errors caused by potentially erroneous decisions of the DFE.
FIG. 2 is a schematic representation of a radio message signal. Each frame 200 typically consists of 6 slots, with each slot containing both the known sequence block 210 (the training symbols) and the unknown sequence block 220 (the data symbols). The training sequence of the North American digital cellular standard contains 14 known symbols for a total of 28 bits. The unknown data block can include 256 symbols (but can be longer or shorter). The choice of how many known symbols to include in waveform is driven by the multipath requirements of waveform. The repetition rate of known symbols (i.e., how far apart or how long unknown data block) is determined by maximum fading (Doppler spread) requirements of waveform.
Even with the application of the most advanced equalizers, demodulating a signal in a multipath and fading environment can be particularly challenging. Most equalizers have some residual intersymbol interference that is ignored since it is considered negligible. As a constellation increases in complexity, the effects of this residual ISI can no longer be ignored and must be addressed. Without accounting for the residual ISI, the equalizer output becomes inaccurate and maybe even useless.
Moreover, for best performance, the received signal must be channel-filtered and channel-matched filtered before it is sent to the equalizer. When the received signal is channel-matched filtered, the channel is spread to both the present and the future symbols (As used herein, future symbols are symbols which are sent later in time relative to current symbol time position.) In other words, additional intersymbol interference is spread to both the known and the unknown data symbols. The conventional equalizers disregard some of this added intersymbol interference. Thus, there is a need for a novel equalizer that accounts for this residual ISI.
An object of the invention is to provide an equalizer architecture to overcome multipath and/or fading caused by the communication channel.
Another object of the invention is to provide a low complexity equalizer adapted to account for the intersymbol interference caused by the channel, the channel-matched filtering, the radio filtering and all other signal filtering operations.
Still another object of the invention is to provide a filtering device to address HF, VHF and UHF propagation bands.